Electrolytic treatment of filamentary carbon material

ABSTRACT

A PROCESS FOR TREATING THE SURFACE OF FILAMENTARY CARBON WHICH COMPRISES MAKING THE FILAMENTARY CARBON THE ANODE OF AN ELECTROLYTIC CELL, EMPLOYING AS THE ELECTROLYTE AN AQUEOUS SOLUTION OF A SOLUE, FOR EXAMPLE IN ALKALI METAL HYDROXIDE, ADAPTED TO GENERATE OXYGEN AT THE ANODE DURING ELECTROLYSIS, AND ELECTROLYSING THE ELECTROLYTE. THE TREATMENT MAY BE CARRIED OUT AS A BATCH PROCESS OR A CONTINUOUS PROCESS.

United States Patent 3,759,805 ELECTROLYTIC TREATMENT OF FILAMENTARY CARBON MATERIAL Dennis R. Chapman, Dufiield, and William Crawford Paterson, Darley Abbey, England, assignors to Courtaulds Limited, London, England No Drawing. Filed Mar. 18, 1970, Ser. No. 20,822 Claims priority, application Great Britain, Mar. 19, 1969, 14,434/ 69 Int. Cl. C01d 7/34; Clllb 13/04; B011: 3/01 US. Cl. 204-130 5 Claims ABSTRACT OF THE DISCLOSURE A process for treating the surface of filamentary carbon which comprises making the filamentary carbon the anode of an electrolytic cell, employing as the electrolyte an aqueous solution of a solute, for example an alkali metal hydroxide, adapted to generate oxygen at the anode during electrolysis, and electrolysing the electrolyte. The treatment may be carried out as a batch process or a continuous process.

This invention relates to the surface treatment of filamentary material and in particular to the preparation of the surface of filamentary carbon to improve its properties for incorporation into matrices.

Filamentary carbon particularly that which is prepared by a process which involves heat treatment at a temperature greater than 1500 C. tends to have a smooth surface which does not bond well into a matrix. Improved bonding may be achieved by modifying the surface to some extent and this invention provides an improved process for such modification.

According to the invention a process for treating the surface of filamentary carbon comprises electrolysing an electrolyte in the form of an aqueous solution wherein the filamentary carbon is connected as the anode such that nascent oxygen is generated at the surface of the filamentary carbon. Thus, a suitable aqueous solution in the process of this invention contains hydroxyl ions and is of a composition such that the hydroxyl ions may be discharged at the anode during an electrolysis to produce nascent oxygen. It is known that not all aqueous solutions containing hydroxyl ions produce nascent oxygen in an electrolysis, for example solutions containing another anion which may be preferentially discharged. Such solutions are not included in the process of this invention.

Carbon filaments suitable for use in the present invention may be produced by subjecting organic filamentary material such as polyester, polyamide, cellulosic or preferably polyacrylonitrile filaments to various conditions of temperature, time and surrounding atmosphere. Carbon filaments are commonly produced for example by heating organic filamentary material in air or other oxidising gas to a temperature within the range from about 200 to 300 C. and the product is carbonised at a temperature above 1000 C. The resulting carbonised filaments may be further heated to build up a graphitic structure within them at temperatures of from 1500 C. upwards. Filaments carbonised at a temperature lower than 1500 C. do not necessarily require surface treatment to make them suitable for incorporation in a matrix but those prepared at a temperature above 1500 C. will normally require such treatment.

Preferably, the electrolysis in the process of this invention is carried out when the electrolyte is an aqueous solution of an alkali metal hydroxide. It is preferred to use sodium hydroxide but the other alkali metal hydroxides are also suitable. The concentration of alkali metal hydroxide in the solution may vary within limits,

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though from the point of view of speed of reaction it may be preferred to use a solution containing from about 10 to 20 percent by weight of the hydroxide.

'In the process of the invention the carbon filaments are made the anode of the electrolytic cell and the cathode is preferably a metal such as nickel which together with the carbon anode produces a cell in which nascent oxygen is produced on the surface of the filaments. Bundles of staple filaments may be treated in a batch process by attaching the whole bundle to a suitable electrical contact terminal and it may be desirable for them to be restrained from floating about in the cell by means of a cell separator. Continuous filaments may be treated by passing them continuously into the electrolytic cell and removing them therefrom and in such a case they may be connected to the source of electromotive force for example by passing them over a conducting roller which itself is connected to that source.

The process of the invention is illustrated by the following examples.

A bundle of fibres of essentially graphitic structure was wrapped in a knitted polypropylene fabric. This polypropylene fabric acted as a cell separator in that it allowed free circulation of electrolyte solution and electric current but prevented loose fibres causing short circuits in the electrolytic cell. The bundle was immersed in a 15 percent aqueous solution of sodium hydroxide. A nickel electrode was also immersed in the sodium hydroxide solution. A current of 2.5 amps at a potential difference of 6 volts was applied for 5 minutes with the nickel as the cathode and the carbon fibres as the anode. The fibres were removed from the cell, allowed to drain, rinsed and dried.

Laminates were prepared using these treated fibres and an epoxy resin and the interlaminar shear strength (a measure of resin/fibre bonding) was found to be superior to that obtained with untreated fibres. Thus, the interlaminar shear strength of laminates prepared with the untreated fibre was 2,700 p.s.i. kg/cm?) and that of laminates prepared with the treated fibre exceeded 7,800 p.s.i. (550 kg/cmf EXAMPLE 2 A continuous yarn of carbon fibres was passed through a bath containing 15 percent sodium hydroxide solution and was made the anode by first passing over a metal conductor roll. Nickel plates attached to the inner surface of the bath acted as cathode. A potential difference of 6 volts was applied between the carbon yarn anode (via the metal roll) and the nickel cathode. This potential dilference gave rise to a current of approximately 2.5 amps under the conditions used. The dwell time of the yarn in the bath was 3 minutes. The treated yarn left the bath through nip rolls to remove excess electrolyte and was then washed and dried and finally wound on to a take-up bobbin. This procedure was carried out using yarns of high tensile carbon fibres and yarns of high modulus carbon fibres. Laminates prepared from the treated yarns had superior resin/fire bonding characteristics to those of the untreated yarn. Thus, the interlaminar shear strength of laminates prepared with treated high tensile fibre was 11,700 p.s.i. (820 kg./cm. and that of laminates prepared with treated high modulus fibre was 8,100 p.s.i. (570 kg./cm. On the other hand,

laminates prepared with untreated yarns of carbon fibres had an interlaminar shear strength of 2,700 p.s.i. (190 kg./cm.

What is claimed is:

1. A process for treating the surface of filamentary carbon to improve its bonding properties in a resin or plastic matrix which comprises making the filamentary carbon the anode of an electrolytic cell, employing an aqueous solution capable of producing nascent oxygen at the anode as the electrolyte in said cell and electrolyzing said electrolyte to generate nascent oxygen at the surface of said filamentary carbon to modify said surface.

2. A process according to claim 1 wherein the solute is an alkali metal hydroxide.

3. A process according to claim 2 wherein the alkali metal hydroxide constitutes from about 10 to 20 percent by weight of the electrolyte.

4. A process for treating the surface of filamentary carbon in the form of continuous filaments to improve its bonding properties in a resin or plastic matrix which comprises making the filamentary carbon the anode of an electrolytic cell, employing an aqueous solution capable of producing nascent oxygen at the anode as the electrolyte in said cell, passing the filamentary carbon continuously into said electrolyte and continuously removing the filamentary carbon from said electrolyte, and electrolyzing said electrolyte to generate nascent oxygen at References Cited UNITED STATES PATENTS 3,323,869 6/1967 Olstowski 23209.1 2,807,577 9/1957 Antonsen 204-430 2,439,442 4/1948 Amon et a1. 23209.1 3,671,411 6/1972 Ray et al 204-130 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. XJR. 204129, 294 

